Accompanying recent popularization of computers, an inkjet printer is widely used for printing letters or drawing an image on paper, film, cloth or the like not only at offices but also at homes.
The inkjet recording method includes a system of jetting out a liquid droplet by applying a pressure from a piezoelectric element, a system of jetting out a liquid droplet by generating a bubble in the ink under heat, a system of using an ultrasonic wave, and a system of jetting out a liquid droplet by suction using an electrostatic force. The inkjet recording ink used therefor includes an aqueous ink, an oily ink and a solid (fusion-type) ink.
Among these inks, an aqueous ink is relatively superior to oily ink or solid (fusion-type) ink in view of possibility of satisfying all of production, handleability, odor, safety and the like and therefore, is predominating as the inkjet recording ink at present.
The colorant used in such an inkjet recording ink is required to have high solubility in a solvent (ink medium), enable high-density recording, provide good color hue, exhibit excellent fastness to light, heat, air, water and chemicals, ensure good fixing to an image-receiving material and less bleeding, give an ink having excellent storability, have high purity and no toxicity, and be available at a low cost. Various dyes and pigments for use in inkjet recording have been already proposed and are actually used, but a colorant satisfying all of the requirements described above is not yet found out at present.
Conventionally well-known dyes and pigments such as colorant having a color index (C.I.) number can hardly satisfy all of those various properties required of the inkjet recording ink.
As for the dye capable of improving the fastness, azo dyes derived from an aromatic amine and a 5-membered heterocyclic amine have been proposed in Patent Document 1. However, these dyes have a problem of bad color reproducibility due to undesirable color hue present in the yellow and cyan regions. Patent Documents 2 and 3 are disclosing an inkjet recording ink with an attempt to enhance the light fastness. However, in use as a water-soluble ink, the colorants used in these patent publications are insufficient in the solubility in water, as a result, satisfactory pictorial quality and gradation cannot be obtained. Also, if the water solubility is high, there arises a problem in the fastness of ink to humidity and heat.
In order to solve these problems, compounds and ink compositions described in Patent Document 4 have been proposed. Furthermore, an inkjet recording ink using a pyrazolylaniline azo dye for improving the color hue and light fastness is described in Patent Document 5. However, these inkjet recording inks are still insufficient to satisfy both the pictorial quality and the fastness of the image output.
With recent spreading of inkjet print, it is strongly demanded to satisfy both the highly refined inkjet image and the image preservability. More specifically, the former requires to enhance particularly the pictorial quality and the latter requires to improve particularly the resistance against heat, light and oxidative atmosphere.
For enhancing the pictorial quality, an inkjet recording method of performing recording by using dark and pale two inks differing in the drawing density has been proposed and is actually used. When multiple inks differing in the kind of dye or in the concentration are used, rich gradation is attained for the halftone of image drawn and the gradation region is also enlarged, as a result, the pictorial quality is enhanced in this point, but in the high density region and the low density region, that is, the shadow part and the highlight part, in many cases, gradation is not reproduced and also change in the color hue is liable to occur, as a result, the pictorial quality is rather deteriorated in this point. It is found that this deterioration of pictorial quality is in a level incapable of correcting by a software for image adjustment of an inkjet printer and must be solved by the ink design.
Out of those various properties required of the inkjet recording ink, it is not yet sufficiently attained to satisfy both excellent image preservability against light, heat and high humidity and excellent pictorial quality of the image drawn, and realization thereof is demanded on the market.
[List of Patent Documents as Background Art]
The above-described technical documents as the background art are as follows.
[Patent Document 1]
JP-A-55-161856 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
[Patent Document 2] JP-A-61-36362
[Patent Document 3] JP-A-2-212566
[Patent Document 4]
JP-T-11-504958 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”)
[Patent Document 5]
Japanese Patent Application No. 2000-80733
An object of the present invention is to solve the problems of inkjet recording ink described above in the item of Background Art, more specifically, to provide an ink set for inkjet recording and an inkjet recording method, which can ensure high pictorial quality of recording image and also excellent preservability of the image obtained.